Inkjet printers have printheads that operate a plurality of inkjets to eject liquid ink onto an image receiving member. The ink may be stored in reservoirs located within cartridges installed in the printer. Various forms of ink include aqueous, aqueous latex, oil, solvent-based, UV curable inks, or ink emulsions. Other inkjet printers receive ink in a solid form and then melt the solid ink to generate liquid ink for ejection onto the imaging member. In these solid ink printers, the solid ink may be pellets, ink sticks, granules, pastilles, or other forms. The solid ink pellets or ink sticks are typically placed in an ink loader and delivered through a feed chute or channel to a melting device that melts the ink. The melted ink is then collected in a reservoir and supplied to one or more printheads through a conduit or the like. In other inkjet printers, ink may be supplied in a gel form. The gel is also heated to a predetermined temperature to alter the viscosity of the ink so the ink is suitable for ejection by a printhead.
A typical full width inkjet printer uses one or more printheads. Each printhead typically contains an array of individual inkjet ejectors for ejecting drops of ink across an open gap to an image receiving member to form an image. The image receiving member may be a continuous web of recording media, a series of media sheets, or the image receiving member may be a rotating surface, such as a print drum or endless belt. Images printed on a rotating surface are later transferred to recording media by mechanical force generated in a transfix nip that is formed by the rotating surface and a transfix roller. In an inkjet printhead, individual piezoelectric, thermal, or acoustic actuators generate mechanical forces that expel ink from a pressure chamber through an orifice in response to an electrical signal, also referred to as a firing signal. The amplitudes, or voltage levels, of the firing signals affect the amount of ink ejected in each drop. The firing signal is generated by a printhead controller in accordance with image data and the firing signal parameters downloaded to the printhead controller. An inkjet printer forms a printed image in accordance with the image data by printing a pattern of individual ink drops at particular locations on the image receiving member. The locations where the ink drops landed are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving member in accordance with image data.
In order for the colors of printed images to correspond closely to the image data, the ink drops ejected onto the media for each ink color should form uniform colors for a given density of the color as specified in the image data. For example, if a region of a media sheet includes a region where 50% of the surface of the sheet should be covered in yellow ink, then the resulting ink image should appear to have a uniform yellow color in the specified region. To achieve the uniform color, the average sizes and masses of individual ink drops that form the ink image should be substantially uniform.
Although known calibration techniques enable initial firing signal parameters to be identified for operating the printheads in the printer to produce uniform colors, the environmental conditions, such as thermal conditions, in most printers eventually affect the actuators in the printheads. Specifically, the actuators begin to degrade, shift, or drift so they no longer eject the same mass of ink that they ejected in response to firing signals generated with reference to the initial firing signal parameters. To compensate for this variation, empirical data have been used to formulate a generic drift curve that identifies a change in a firing signal parameter with some parameter, such as the amount of time a printhead is exposed to temperatures in a predetermined range. Then, the time of such thermal exposure is monitored for each printhead and when a printhead accumulates a period of thermal exposure that noticeably affects the mass of the ink ejected by a printhead, one or more firing signal parameters are adjusted by an empirically determined change value or values. These empirically determined change values, which typically increase a firing signal parameter, operate the actuators in the printhead to eject ink masses that are in an acceptable range about the initial ink drop mass.
While this known technique can be effective, it sometimes fails to bring all printheads in a printer within the acceptable range. This failure is attributed to variances in the printheads. For example, all printheads do not follow the empirically determined thermal drift curve. Consequently, adjusting firing signal parameters for one printhead may overcompensate for changes that have occurred in the printhead performance, while the same adjustment for another printhead may under-compensate for changes in that printhead's performance. Thus, techniques that better adjust printhead parameters while maintaining uniform ink drop mass are desirable.